The study of sensory systems provides an opportunity to observe the stunning complexity of the nervous system. Even Darwin himself once remarked that the organization of the eye was so intricate that the odds of it arising from natural selection seemed almost “absurd in the highest possible degree.” Dr. Chih-Ying Su at UCSD focuses her research on the functional organization of olfactory systems in an effort to determine how olfactory receptor neurons (ORNs) are able to process a staggering range of different odors without requiring an infinite number of ORNs to do so.
Dr. Su carries out her research in fruitflies and mosquitoes. In such insects, ORNs are compartmentalized into units called sensilla. Although ORNs are thought to respond to specific odorants independently, the functional significance of their compartmentalization into these stereotyped groups is unknown. Dr. Su investigated this organization by analyzing the relationship between pairs of ORNs within a given sensillum. To do this, she would expose the ORNs to an odorant that would elicit consistent firing of one neuron, and then intersperse bursts of a second odorant to produce firing from the second neuron.
How do two ORNs behave when they are both activated? As it turns out, it appears that the activation of an ORN has an inhibitory effect on neighboring ORNs. Thus, if neuron A is exposed to an odorant consistently and then a second odorant is introduced, the activation of neuron B reduces the firing of neuron A. To rule out the possibility that the second odorant acts directly on neuron A to suppress its firing, the same assay was carried out following ablation of neuron B. Without communication from its neighbor, neuron A failed to demonstrate inhibited firing. The same results were demonstrated when the roles of the neurons were reversed (i.e. neuron A can also inhibit the sustained firing of neuron B), as well as when the neurons were activated by Channelrhodopsin2 rather than by an odorant stimulus.
This phenomenon is known as lateral inhibition. Interestingly, Dr. Su observed that lateral inhibition between ORNs within a sensillum does not rely on classic synaptic transmission. In fact, it doesn’t require synapses at all. She expressed tetanus toxin (TNT) specifically in ORNs to silence synaptic transmission. Even in these genetically-modified flies, lateral inhibition of a similar degree to control flies was observed. Based on Dr. Su’s characterization of these neurons, it appears that the ORNs communicate via ephaptic transmission, which is a special kind of communication that occurs between adjacent neurons via an extracellular electrical field.
The identification of the functional significance of insect sensilla is particularly intriguing because it provides insight into how odorant mixtures are processed. This is a unique approach to the study of olfaction, as many researchers commonly use only single odorants in their experiments. While that is certainly useful, this two-odorant approach allowed Dr. Su to simulate a more life-like scenario inside the laboratory. By virtue of this, her research could give way to novel methods of insect control in the real world. Perhaps it will soon be possible to utilize odorant mixtures to suppress specific ORNs that would normally drive insects towards a specific target via activation of their neighboring ORNs.
To learn more, check out Dr. Chih-Ying Su’s talk in the CNCB Marilyn Farquhar Seminar Room at 4pm on April 26, 2016.
Caroline Sferrazza is a first year student in the UCSD Neurosciences Graduate Program. She is currently rotating in Dr. Rusty Gage’s lab at the Salk Institute where she uses stem cells to study Bipolar Disorder. When she has a chance to ditch the lab coat, she can generally be found anywhere there’s good music or cute animals. Preferably both.